III.2.2 Level 2 Subdomain
If a larger network is needed, requiring level-2 routing, then the level-2 subdomain forwards packets between the level-1 areas, and thus must support all of the types of packet present in all of those level-1 area. The rules for the level-2 subdomain are:-
1. If IPv4 packets are forwarded in any of the areas (IP-only or Dual areas), then all of the routers in the level-2 subdomain must be able to forward IPv4.
2. If CLNP packets are forwarded in any of the areas (OSI-only or Dual areas), then all of the routers in the level-2 subdomain must be able to forward CLNP.
Therefore, if any of the areas are dual, or if both OSI-only and IP-only areas exist, then, the routers in the level-2 subdomain must be dual. This is illustrated in Figure III-2.
Figure III-2
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